This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
It is rather common for a device in a communication network, such as a computer network, to want to know the “distance” to another device, usually to determine if the other device is in the proximity.
One way of measuring distance is to use Round Trip Time (RTT) measures. The device sends a message, requesting the other device respond directly, and starts a timer that runs until a response is received from the other device. The time is known as the Return Trip Time. Often, the RTT is calculated as an average over several measurements.
WO 99/059304 presents another way of measuring distance through the use of Time-To-Live (TTL) in messages. The sender includes an initial TTL value in a message. Each node, also known as router, that forwards the message decrements the current TTL value. When the value reaches zero, the message will not be forwarded any more. While this way does not enable direct measurement of the distance counted as a number of “jumps” or “forwardings”, it is possible for the sender to request to recipient to return the TTL value in the received message. Naturally, if the recipient is too far away, it will normally not receive the message at all. A typical initial TTL value (usually called “TTL ceiling”) is three.
A problem with TTL is that the TTL value is not secure: it is easy to increase the TTL value so that the message may be sent further than intended. In addition, the recipient may then simply respond that the TTL value of the received message is 3, 2, 1, or 0. The sender will then assume that the recipient is within the required distance.
A basic solution to the problem would be to cryptographically sign the TTL value in the message so that it cannot easily be changed to a higher value. This would, however, require that all (or, at the very least, most of) the nodes in the network support the signing of the TTL values, which is not always practicable.
The present invention attempts to remedy at least some of the concerns connected with Time-To-Live in the prior art, by providing a method for distance calculations using Time-To-Live, which increases the security of the calculations.